1. Field of the Invention
The present invention relates to an interference power estimating device and an interference power estimating method.
2. Description of the Related Art
An HSDPA (High Speed Downlink Packet Access) system is a communication system of which a status is given as a next generation version of a W-CDMA (Wideband Code Division Multiple Access) system. The HSDPA involves utilizing an adaptive modulation system for scheming to increase a communication speed. In the adaptive modulation system, a best modulation system is automatically selected corresponding a communication environment.
In the communication system using the adaptive modulation system such as the HSDPA, each of terminals transmits a CQI (Channel Quality Indicator) (quality of reception) based on an estimated value of SIR (Signal to Interference power Ratio) to the nearest base station. The base station selects a transmission format corresponding to the quality of reception of which the terminal notifies, thereby realizing a high throughput. Accordingly, this type of communication system requires each terminal to perform the SIR estimation with high accuracy.
Further, DPCH (Dedicated Physical CHannel) utilized in the W-CDMA is allocated individually to each terminal, whereby items of control information such as a pilot symbol and a TPC (Transmit Power Control) bit and user data are transmitted. The DPCH enables high-speed transmit power control (TPC) to be applied. In the DPCH, a degree of how much the transmission power is increased and decreased is determined based on a result of the SIR estimation, and hence accuracy of the SIR estimation exerts large influence on a quantity of traffic that can be handled by the system.
This type of SIR estimation involves estimating reception power and interference power, and an SIR value is obtained from a ratio of the reception power to the interference power that are output based on their estimations. The interference power is estimated generally by a method as illustrated in FIG. 7. FIG. 7 is a conceptual diagram showing an interference power estimating method according to the related art. FIG. 7 illustrates an example in which the interference power in a slot depicted by oblique lines in CPICH (Common Pilot CHannel) is an estimation target.
The conventional interference power estimating method involves calculating noise power about each piece of pilot symbol data allocated to the estimation target slot, calculating an average value of the noise power about the estimation target slot, and outputting the average value as interference power of the estimation target slot. The noise power about each pilot symbol is, according to the example in FIG. 7, obtained by power-valuing a difference between each pilot symbol and the average value of the pilot symbols (the reference signal) for 1-slot time with each pilot symbol being centered.
FIG. 8 is a diagram illustrating function blocks of the conventional interference power estimating circuit. In the conventional interference power estimating circuit, when the pilot symbol data is inputted, a reference signal calculating unit 221 extracts the reference signals for 1-slot time with the inputted pilot symbol data being centered, and acquires an average value of the reference signals. Subsequently, a subtracter 222 obtains a difference between each piece of the inputted pilot symbol data and the average value of the reference signals, then a power-valuing unit 223 squares the difference value, and the squared value is output as the noise power.
With respect to the target slot, the noise power about the respective pieces of pilot symbol data output from the power-valuing unit 223 is added up, and the added noise power is divided by the number of symbols in the target slot, thus, the average value of the noise power is calculated (an averaging unit 224). The thus-calculated average value of the noise power is multiplied by a correction coefficient (multiplier 225), thereby, an interference power value of the target slot is obtained.
The correction coefficient is a value to be multiplied in a way that takes account of an error between the reference signal average value calculated by the reference signal calculating unit 221 and an ideal value. Namely, the correction coefficient is a value for correction the bias, wherein the bias is caused by a result that the reference signal is biased to the target pilot symbol data because the target pilot symbol data is equal to one symbol in the reference signal. The example in FIG. 7 shows that one slot consists of 10 pilot symbols, an average value of the 10 pilot symbols are utilized, and therefore the correction coefficient becomes 10/9.
In this type of conventional interference power estimating circuit, if a CDMA communication system is utilized for obtaining the pilot symbol data, the pieces of pilot symbol data are RAKE-received in order to reduce multi-path interference. The RAKE-reception is a technique for increasing reception power by separating each delay wave from the signals overlapped with the delay waves due to the multi-paths and synthesizing after conducting synchronous detection. The synchronous detection involves using a channel estimation value for adjusting phases of the respective delay waves.
The channel estimation value is, as illustrated in FIG. 9, calculated from the target pilot symbol and an average (a long-term average (movement average)) of a predetermined number of pilot symbols with the target pilot symbol being centered. FIG. 9 is a diagram showing a method of outputting the reference signal (a first reference signal) according to the related art. FIG. 9 illustrates the method of outputting the first reference signal for obtaining a noise component related to the head pilot symbol in the target slot shown in FIG. 7. The first reference signal in FIG. 9 represents the reference signal that is referred to for obtaining the interference power illustrated in FIG. 7.
The example in FIG. 9 is that the reference signal (which will hereinafter be termed a second reference signal) for 1-slot time with each data symbol being centered, is used for outputting the respective data symbols of the first reference signal. FIG. 10 is a diagram showing function blocks of a conventional reference signal outputting circuit.
In the conventional reference signal outputting circuit, when the received signal is despread (a CPICH despreading unit 241) with a spreading code for CPICH at timing corresponding to each path, the despread pilot signal corresponding to each path is synchronously detected (a synchronous detection unit 243). The synchronous detection unit 243 takes phase-synchronization of the pilot signals corresponding to the respective paths by multiplying the pilot signals corresponding to the respective paths by the channel estimation values, corresponding to the respective paths, calculated by the channel estimation value calculating unit 242. Thus, a RAKE-synthesization unit 244 synthesizes the pilot signals that have been phase-synchronized, thereby outputting the pilot symbols.
The channel estimation value calculating unit 242 calculates the channel estimation value by use of the average value of the pilot symbols (the second reference signal) for 1-slot time with each pilot symbol being centered in order to synchronously detect each of the pilot symbols.
The conventional interference power estimating method has, however, a problem that fluctuations of the signal components in a fast fading environment are treated as noises. A noise increment derived from the fluctuations of the signal components does not become a factor for a decline of demodulating capability.
Accordingly, the execution of the SIR estimation with high accuracy entails realizing the interference power estimating method capable of the estimation in a way that does not treat the fading fluctuations as the noises.